Is there a way to symmetrize a mesh without messing up the armature underneath?
Alright so I figured it out so I figure I'll update just in case anyone ever finds themselves stuck in my situation. Turns out the fix was very obvious and simple after all. :)
The thing that fixes the symmetry in a way that isn't destructive is actually 'mesh &gt; snap to symmetry' rather than using symmetrize. This will preserve not just the armature but all of the weight painting, too. You can either select the entire mesh or just the opposing areas on each side of the mesh that you want symmetrical and you will get this wonderful popup.

I guess prevention is always best but I'm really relieved that it's not the end of the world (and all of my weight painting) if the mesh accidentally loses symmetry.